Label‐Free Imaging of Low‐Index Samples Using Substrate‐Enhanced Subwavelength‐Resolution Scattering Nanoscopy

There is an increasing demand for label‐free imaging of subwavelength low‐index dielectric samples with high contrast and high sensitivity using a dark‐field optical microscope. However, subwavelength low‐index dielectric samples usually do not show strong Mie scattering resonance characteristics due to the weak light confining ability, which makes them difficult to clearly resolve under a traditional optical microscope. In this work, a fast, simple, and effective approach is proposed to enhance the optical response of subwavelength low‐index dielectric samples by using metallic substrate‐enhanced scattering nanoscopy. It is found that the high reflectance of a substrate can significantly enhance the electric dipole resonance of a SiO2 nanoparticle and the substrate‐nanoparticle interaction, which amplifies the scattering signal of the sample and improves the imaging contrast. Compared with the silica substrate, the Ag‐coated substrate can enhance the scattering intensity of a SiO2 nanoparticle up to 3.7 times under 540 nm wavelength illumination. Furthermore, label‐free stand‐alone 50‐nm‐radius SiO2 nanoparticles and ≈100‐nm‐thick bacterial capsules can be clearly observed when placed on Ag‐coated substrates under a conventional dark‐field optical microscope. The substrate‐enhanced subwavelength‐resolution scattering nanoscopy can enhance the scattering of low‐index subwavelength samples and benefit a significant application in biology, medical testing, and optical detection. A simple, fast, and effective metal layer substrate is prepared that improves the electric dipole resonance of the nanoparticles. Compared with a silica substrate, the scattering intensity of SiO2 nanoparticles is enhanced by ≈3.7 times under 540 nm wavelength illumination and a bacterial capsule ≈100 nm thick is successfully observed. Substrate‐enhanced scattering nanoscopy has potential implications in super‐resolution, high‐contrast, label‐free imaging of low‐index samples.